Freestyle multi-color per object?
I would suggest using the Material modifier for the freestyle edges.

Apply different materials to the object for the parts you want to color.



To control a property, use the Material modifier and select the material property.



Select the Freestyle Line color as you wish. I'll use the Diffuse Red channel to control the thickness and the Diffuse Blue channel to control the Opacity.

Note that the color channels can have any float value.



The green material has a Priority of 1.